The Wynn Dyson imaging system has been used in the manufacture of semiconductor devices for approximately 30 years. The basic Wynn-Dyson imaging system has a reflective mirror and a refractive doublet located in the vicinity of the mirror focus.
One limitation of this fundamental design when used for photolithography is that a large reticle in the object plane would interfere with a large wafer residing in the image plane as the reticle image is “step and repeated” across the wafer to fully expose the entire wafer.
The basic Wynn-Dyson imaging system has thus been modified for use in photolithography by using prisms attached to the doublet assembly. The prisms serve to move the object and image planes into non-conflicting planes. Example Wynn-Dyson imaging systems as modified for use in photolithography are disclosed in U.S. Pat. Nos. 6,863,403, 7,116,496, 6,813,098, and 7,148,953.
FIG. 1 shows a prior art Wynn-Dyson imaging system 8. The Wynn-Dyson imaging system 8 has a concave mirror 10 disposed along an optical axis 14. Mirror 10 has an aperture stop AS that serves to define a numerical aperture (NA) for the system. A lens system 13 is arranged along optical axis 14 and is spaced apart from mirror 10. The lens system 13 has a front end 13F that faces mirror 10, and a rear end 13R that faces away from mirror 10. The lens system 13 includes a meniscus lens 13A at the front end 13F of the lens system, and a plano-convex lens 13B at the rear end of the lens system.
Wynn-Dyson imaging system 8 also includes two prisms 17 and 19 that define a prism assembly. Prisms 17 and 19 are each arranged on opposite sides of optical axis 14, with one of their surfaces in intimate contact with respective portions of the planar surface of plano-convex lens 13B, which defines the rear end 13R of the lens assembly 13. A reticle 16 resides in an object plane OP and a wafer 18 resides in an image plane IP. Prism 17 resides adjacent reticle 16 while prism 19 resides adjacent wafer 18.
A typical photolithography reticle 16 has transmissive regions and opaque regions. The opaque regions are typically made of chrome, which reflects approximately 70% of the UV light 11 incident thereon, and absorbs the remaining 30%. The transmissive regions are clear and transmit UV light 11 with little absorption. For reticles that are 50% transparent, approximately 15% of the total UV light incident thereon is absorbed by the reticle. In a typical photolithography system, the power level of the UV light 11 incident on reticle 16 is about 2 watts/cm2, which implies that approximately 300 mw/cm2 of light is absorbed by the reticle. In some conditions, a reticle might be only 10% transmissive, in which case, approximately 540 mw/cm2 of light is absorbed by the reticle. This is sufficient to cause heating, and subsequently, mechanical distortion in the reticle.
The UV light 11 from the illuminator (not shown in FIG. 1) that is absorbed by the reticle 16 heats the reticle, which is in close proximity to prism 17. Measurements by the inventors have shown that, in some cases, reticle 16 can heat to up to 50° C. when irradiated by UV light 11. This heating causes two main problems. The first is that the reticle itself distorts. The amount of reticle distortion depends upon the type of glass used for the reticle. However, typical values for the coefficient of thermal expansion (CTE) for common reticle materials such as quartz is about 1 part per million (ppm) per ° C. Hence, a 30° temperature rise can distort the reticle by approximately 30 ppm. For a 36 mm long image field at the wafer, this distortion is calculated as (36 mm/2)·30×10−6=0.5 microns. Depending upon the lithography and overlay requirements, this may be highly problematic.
The second main problem is that the heat from the reticle is transferred to the adjacent prism 17. It has been observed by the inventors that this heat transfer can cause prism 17 to bend, which leads to an asymmetric image distortion at the wafer (i.e., the reticle image is distorted).
FIG. 2 is a close-up schematic diagram of reticle 16 and the adjacent prism 17 according to the prior art configuration of FIG. 1. Prism 17 has a proximal surface 17a and a corner or tip 17c. As the reticle 16 heats up, it heats the adjacent prism 17 by convective heating and by radiative transfer of heat (the reticle emits heat in the infrared, e.g., around 10 microns in wavelength). The convective heat or convective heating is represented in FIG. 2 by dashed lines and denoted 20C, while the radiative heat or radiative heating is represented by arrows and denoted 20R. The overall heating is denoted as 20. The tip 17c of the prism 17 heats up more than its base and so bends. This bending causes the image of the reticle 16 at the wafer 18 to shift in one direction, which is to say that the reticle image suffers from thermally induced distortion.
Prism bending changes the distortion (or magnification) in the “y” direction (i.e., creates a “Y-mag” change). Under some conditions (e.g., where the reticle is 95% chrome, and hence, absorbs approximately 600 mw/cm2), the steady-state Y-mag change can be over 50 ppm, thereby leading to over 1 micron of image distortion at the wafer (image) plane. This directly leads to a 1 micron overlay error.
Generally, it is required that the overlay accuracy of a photolithography system be approximately 25% of the linewidth being printed. A 1 micron overlay error implies that the smallest feature that the photolithography system can be used to manufacture would be about 4 microns, independent of the resolution of the system. Hence, even if the photolithography system has a resolution of 1 micron, it cannot be used in manufacturing for features smaller than 4 microns.
Prior attempts to solve the problem of thermal distortion of the prism include flowing cool air between the reticle and the prism. Unfortunately, the time-dependent nature of the prism heating renders this approach unsatisfactory. When the photolithography system is sitting idle for more than a few minutes, the prism 17 returns to its base temperature. When the photolithography system is then operated, the prism 17 begins to heat up. The temperature of the prism 17 increases with increasing number of wafers 18 processed.
However, after only one wafer being processed, the reticle temperature typically increases by only a few ° C., while after 10 wafers, it typically increases by 20° C. to 30° C. Therefore, the reticle temperature (and hence, the prism temperature) is time dependent. Unfortunately, air cooling usually ends up overcooling the reticle initially during the first few wafers and then does not adequately cool the reticle for larger numbers of wafers (e.g., 10 or more). In other words, the time dependency of air cooling is too slow relative to the heating profile of the photolithograph system to make air cooling an effective solution for reducing thermally induced distortion. Furthermore, the air flow approach only mitigates convective heating and does not mitigate radiative heating.